Electrolytic gold baths are used for many applications. They generally contain alkali or ammonium gold (I) cyanide or, less commonly, gold(III)-cyanides or alkali gold sulfites.
Gold baths for decorative applications almost always deposit gold alloys with considerable amounts of alloy metal to obtain desired color effects.
Gold baths also are used widely to obtain gold with good electric and mechanical properties for use in lightduty electric contacts. These baths operate in a weakly acidic pH range (pH 3-5). The coating contains usually small amounts (0.1-1%) of nickel, cobalt or iron in addition to gold.
Moreover, electrolytic gold baths are also used for the deposition of fine gold layers with a gold content of at least 99.9 %, e.g. as bondable coatings in semiconductor technology.
The term bonding techniques refers to those methods in which system carriers are connected in a conductive manner in microelectronics to electronic components, e.g. chips, via fine wires (consisting usually of gold or aluminum). The connection of the wire to a gilded connection surface takes place by means of pressure, elevated temperature and is frequently supported by ultrasonic energy.
The bonding connection is only successful if the gold coating on the connection surface is very pure (fine gold with a gold content of at least 99.9%), soft (maximum hardness 120 HV) and satin-finished. Hard, highly lustrous coatings are unsuitable. Further requirements for bondable gold layers result from the stressing from heat-stress tests which are carried out to assure a good bondability. Such heat-stress tests are carried out e.g. on an unbonded test piece. No discoloration of the gold layer is allowed to occur in these tests e.g. after heating for 5 minutes at 500.degree. C. in air. In other tests, the bonded test piece is exposed to temperature-change tests or to a temperature of 150.degree.-180.degree. C. for many hours in air. These heat-stress tests bring about diffusion processes between the gold and the carrier material in the gold layer. The effect of such diffusion processes on the bond behavior is a function of the thickness of the layer of the gold and of its structure. Since it is required that the gold layer be as thin as possible, for economic reasons, the structure of the coating must be optimized by means of bath additives and the selection of suitable deposition conditions in such a manner that a minimum thickness of the gold layer assures acceptable bondability. Such bath additives have previously been known in the form of arsenic, thallium or lead.
A further criterion for the economy of a electrolytic method is the deposition speed of the bath, consistent with obtaining the desired properties in the gold layer. The deposition speed results from the usable current density, whose upper limit should be as high as possible without loss of the desired properties of the gold layers.
Simple possibilities for obtaining high, reliable current densities are the elevation of the gold content in the bath or a rapid electrolyte motion. On the other hand, because of the high price of gold, the economy of the method suffers if the gold contents are too high.
The use of apparatus (special electrolytic cells) which provide very high electrolyte motion by flowing the bath or spraying it against the material to be plated often also selectively limits the deposit of gold onto the functionally important surface elements of the item, by means of the use of masks. However, a strong flow through the electrolyte is important for the speed of deposition (maximum permissible current density).
In spite of the elevation of the deposition speed which can be achieved with these measures, a further increase of current density and of deposition speed by means of improving the composition of the gold baths is very desirable. Furthermore, increased requirements are placed on the stability of the electrolyte as a result from the high current density to be used (cathodic and especially also anodic) as well as from the strong turbulence of the baths in flow systems.
Published European Patent Application EP-OS 0 126 921 describes a bath for the electrolytic deposition of gold alloys which also contains between 10 mg and 100 g/l bismuth in the form of a water-soluble complex compound in addition to alkali gold (I)-cyanide and phosphonic acids. This bath, which operates in a pH range of 6 to 13, forms rose to violet-colored alloy coatings for decorative purposes with gold contents of 65 to 85% by weight which are totally unsuited for bonding applications.
Special gold layers are obtained from electrolytic baths containing hydrogen phosphates, phosphonic acids and nitrogenous carboxylic acids in addition to alkali gold (I)cyanide (Published German Patent Application DE-OS 35 37 283). Current densities up to 15 A/dm.sup.2 can be obtained by this means in flow systems. However, dull brown coatings which are useless for bonding applications are obtained at normal bath motion at current densities as low as 1 A/dm.sup.2.
U.S. Pat. No. 3,879,269 describes gold baths for the deposition of bondable fine gold coatings in high-speed cells. These baths contain a critical amount of 2-12 mg/l trivalent arsenic ions in addition to 24-40 g/l gold, phosphates and carboxylic acids. In spite of their use in a high-speed cell and in spite of a very high gold concentration, useful coatings are achieved only at current densities up to approximately 4 A/dm.sup.2.
Electrolytic gold baths with trivalent arsenic as a grain-refining additive all suffer from the known phenomenon that the trivalent arsenic is oxidized to a pentavalent arsenic, which process occurs even when the bath is not used, but at an especially high reaction speed under the oxidative influences (high anodic current density, strong air flow) of high-speed electrolysis in flow or spray cells. Pentavalent arsenic exhibits no grain-refining or luster-forming effect. On the other hand, the required and acceptable concentration of trivalent arsenic is in a range of some mg/l. Since a sufficiently precise method of analysis for this active component is not available, a reliable bath control is not assured. The instability of the additive results in significant variations in quality which can be limited in an unpredictable manner only by means of frequent function tests and corrections.
Published German Patent Application DE-OS 33 41 233 relates to an acidic fine gold bath with an addition of 5 to 50 ppm (mg/l) lead which can be operated in a current density range of 0.5 to 2 A/dm.sup.2. The optimal current density is 0.6 A/dm.sup.2.
The use of thallium as a grain refiner in concentrations of 1-140 mg/l in gold baths with a pH in the range of 7-13 is described e.g. in Published German Patent Application DE-OS 21 31 815. The useful current density range is 0.1-20 A/dm.sup.2.
It is known in the case of electrolytic gold baths containing lead or thallium as grain-refining additives that gold coatings deposited out of them are unobjectionably bondable in a freshly deposited state; however, they loose their resistance to rupture to a large extent in the subsequently performed heat tests of e.g. 150.degree. C./ 24 hours, so that bond tearing occurs.